The present invention relates to lens meters for measuring the spherical power, the cylindrical power, the cylindrical axis angle and the prism power of an ophthalmic lens.
Examples of lens meters of this type are a telescopic lens meter in which the main optical system is arranged along a straight line and the lower portion of the body holding the optical system is formed as a stand with which the lens meter can be supported on a table or bench and a projection type lens meter in which an enlarging projection system is employed for the observing section. Both of these lens meters are so designed to be used on a table or bench.
These conventional lens meters are disadvantageous in the following points. As they are heavy and bulky, these lens meters are inconvenient for the optician in the case where he must visit a customer for the measurement of ophthalmic lenses. Even in his shop, he can use them only at their particular installation locations. Furthermore, since the conventional lens meters' operating members are arranged to be operated with the meter set on a bench, it is impossible to perform measurements with the lens meter held in one hand.
FIG. 1 shows essential components of a conventional telescopic lens meter. The lens meter has an optical system including a light source 1, a target 2, an objective lens 3, a relay lens 4, a focusing screen 5 and an eyepiece lens 6 all of which are arranged along a single straight line. A lens 8 to be measured is held on a lens rest 7 with a lens holder 9 urged elastically. Provided on the side of the body 10 are a power-measuring dial 11 adaphted to move the target 2 in the direction of the optical axis and a knob 13 for controlling the position of a lens supporting plate 12 adapted to support the lens 8 to be examined. Furthermore, a power-indicating optical system (not shown) is provided in the body 10, which is used to read through the eyepiece lens 6 a lens power scale which is operated in association with the target 2.
The operation of the conventional telescopic lens meter thus constructed will be described.
A lever 14 integral with the lens holder 9 is lifted to place the lens 8 to be measured between the lens rest 7 and the lens holder 9. Under this condition, the knob 13 is turned so that the lens 8 is held with the lens supporting plate 12 abutted against the edge of the lens. Then, the dial 11 is turned to move the target 2 back and forth so that the image of the target 2 is formed on the focusing screen 5. The graduations and the configuration of the target 2 are observed through the eyepiece 6 to measure the power, the orientation of cylinder axis and prism power of the lens 8.
As is clear from the above description, in the conventional lens meter, the main optical system thereof is an aligned type and an optical system for indicating the lens power is also incorporated into the instrument. With this construction, it is impossible to operate the various operating members with the body held in one hand. Furthermore, the conventional lens meter cannot easily be transported as it is bulky and heavy. Thus, the conventional lens meter is disadvantageous in that it is difficult to perform various operations with the lens meter held in one hand. A projection type lens meter also suffers from the same difficulties because the image is enlarged by increasing the focal length of the relay lens 4 and the optical path from the relay lens 4 to the focusing screen 5.